The present invention is directed towards a method for determining a reliability and its associated inherent safety for a rotating component over a range of speeds.
In many machines having rotating components, such as aircraft, the speed of the rotation will have an affect on the resulting reliability of the rotating component such that at faster speeds the rotating component has a higher likelihood of failure. In order to address this issue, and to prevent in flight failures, the FAA has implemented flight safety substantiation regulations which require a manufacturer to demonstrate the reliability of a certain rotating component, before the rotating component can be approved for use.
One factor which can lead to damage of a rotating component, is if the rotating component is operating in an overspeed condition. All rotating components have a rated speed at which they are able to properly operate. Often, a rotating component will be capable of rotating at a faster speed than the rated speed without becoming damaged. When a rotating component rotates at a speed above a threshold percentage of the maximum rated speed, the rated speed the rotating component is described as operating in an overspeed condition. The threshold percentage is determined on a per application basis. By way of example a threshold speed for an auxiliary power unit may be set at 103% of the rated speed, and therefore any rotation speed above 103% would be considered to be an “overspeed” condition.
An uncontained burst 514 can result from a few known processes as illustrated in FIG. 7. The first process occurs when a component speed increases beyond the maximum rated speed without the occurrence of an automatic shutdown 502. The occurrence of both an overspeed condition and the component failure probability being exceeded 504 is indicated in the figure by way of an AND gate 516. When both conditions occur, an elevated energy fragment release 510 is likely. When an elevated energy fragment release 510 occurs it is possible for the fragment to break through any containment and impact on other components, resulting in an uncontained burst 514.
A second process which can result in an uncontained burst 514 occurs when there is a high energy fragment departure 506, potentially due to wear. If the high energy fragment departure 506 occurs at the same time as a containment part failure 508, as indicated by the AND gate 518, a high energy fragment release 512 results. Both the first process and the second process result in an uncontained burst 514, as the fragments from the burst have not been properly contained by a containment component. The occurrence of either an elevated energy fragment release 510 or a high energy fragment release 512 resulting in an uncontained burst 514 is indicated by an OR gate 520.
One factor in failure rates is the strength of the material from which the rotating component is constructed. Since the strength of a material is a factor in the rotating component reliability, the material strength is utilized in the reliability calculations. It is known in the art that the material strength will vary between a set range of strengths for any material used. While an average material strength can be used to determine a rotating component reliability value, a more accurate reliability value is obtained when the entire range of material strengths is considered.
Current methods and techniques in the art for demonstrating reliability of a rotating component additionally neglect to account for the affect of a varying speed on the reliability or the affects of varying material strengths, and instead determine a single static reliability value based on an average material strength and average speed. One affect of the failure to account for varying speeds is that the reliability issues associated with overspeed conditions are largely ignored in the current state of the art.